If the human footprint exceeds the ecologically sustainable footprint for a long period of time, it exhausts many natural resources and thus reduces the sustainable footprint much further.  Think a mass extinction event including many species, but also a large part of humanity.  Plus - the political instability created by much increased competition over ever diminishing resources will probably result in an Armageddon style nuclear war sooner or later, much reducing the ongoing sustainable footprint still further.  Many species are far more adaptable than humans, and humans can often only adapt to environments where they can control the parameters within which that environment can fluctuate.  We can build sea walls but thy can only contain rising sea levels up to a point... and the intensification of agriculture is heavily oil dependent.

Index of Frank's Diaries